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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

294

TIN-PLATINGóZINC-PLATINGóLEAD-PLATING

TIN-PLATING

The surface of the object or scrapings from it are treated with hydro-
chloric acid diluted with an equal volume of water and gently heated. The
liquid is filtered and treated with a drop of mercuric chloride solution, a
white or grey precipitate of calomel or metallic mercury being formed in
presence of tin.

ZING-PLATING

The surface is heated gently with dilute sulphuric acid and the solution
transferred to a beaker and treated with hydrogen sulphide. The filtered
liquid is freed from the excess of hydrogen sulphide, treated with a little
ammonium chloride, rendered faintly alkaline with ammonia, boiled, and
again filtered. To the filtrate, acidified with acetic acid, potassium ferro-
cyanide is added. In presence of zinc, a dirty white flocculent precipitate
forms either immediately or after some time.

LEAD-PLATING

Scrapings of the surface are treated in a dish with nitric acid, evaporated
to dryness and taken up in a few drops of water. The solution is tested
for lead by means of potassium chromate or iodide.

ALUMINIUM-PLATING

The surface, or scrapings of it, are heated with 10% sodium hydroxide
solution. The Hquid is diluted somewhat, filtered, acidified with hydro-
chloric acid and made alkaline with ammonia. A white gelatinous pre-
cipitate is formed in presence of aluminium.

COPPER-PLATING
(on Iron)

The object or part of it is treated in the cold with cone, nitric acid,
which dissolves the copper but scarcely affects the iron. The solution is
decanted off, diluted and tested for copper with ammonia.

BRASS-PLATING

The surface is treated with nitric acid diluted with an equal volume of
water, the liquid being filtered and rendered alkaline with ammonia : the
characteristic blue coloration of copper in ammoniacal solution is observed.
The liquid is then acidified with hydrochloric acid, treated with hydrogenal of methylamine hydrochloride being